Extraordinary adaptation to aridity in a disappearing fauna of Mexican vertebrates

Deserts represent a challenging environment for many organisms. Organisms are expected to adapt to decreasing precipitation through decreasing dependence on water. Here, we reveal a surprising alternative pathway to surviving arid climates. We examine phylogenomic and ecological data for four reptile genera, each containing an endemic species in the Cuatro Ciénegas Basin in the Chihuahuan Desert of Mexico. Each endemic is morphologically distinct but phylogenetically nested inside species that occur in more mesic environments. We find that three endemics are exceptionally young, including the youngest known squamate species (only ~76,000 years old). These endemic species show rapid rates of climatic-niche evolution, from 10–112 times comparable rates among their close relatives. These four endemics underwent habitat shifts, from being ancestrally terrestrial, non-riparian species to become strictly riparian. Thus, they became more dependent on water, rather than less. Remarkably, three species also rapidly shifted from being primarily diurnal to primarily nocturnal, and from being most active during cooler months to being most active during the hottest (but wettest) months. We also found that many riparian reptile species (11/27; 41%) may have recently disappeared from the Cuatro Ciénegas Basin, including an endemic species that may now be globally extinct.